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USNhf: 



THE SCIENTIFIC 
MONTHLY 



NOVEMBER, 1922 



SOCIAL LIFE AMONG THE INSECTS 1 

By Professor WILLIAM MORTON WHEELER 

BUSSEY INSTITUTION, HARVA*KJjfA-NIVEBSITY 

LECTURE IV ANTS, THEIR DEVELOPMENT, CASTES, NESTING AND 
~"~~ FEEDING HABITS 

ON one occasion several years ago when I was about to lecture 
on ants in Brooklyn, a gentleman introduced me to the audi- 
ence by quoting the sixth to eighth verses of the sixth chapter of 
Proverbs, and then proceeded in utter seriousness to give an inti- 
mate account of their author. He said that Solomon was the 
greatest biologist the Hebrews had produced, that he had several 
large and completely equipped laboratories in which he busied 
himself throughout his reign with intricate researches on ant 
behavior and that the 700 wives and 300 concubines mentioned in 
the Bible were really devoted graduate students, who collaborated 
with the king in his myrmecological investigations. The gentleman 
deplored the fact that the thousand and one monographs embody- 
ing, their researches had been lost, and concluded by saying that 
he was delighted to introduce one who could supply the missing 
information. As he had consumed just forty-three minutes with 
his, account of Solomon and his collaboratrices, I had to confess my 
inability to "deliver the goods" in the remaining seventeen! From 
what recondite sources of biblical exegesis the Brooklyn gentleman 
drew his information I have never been able to ascertain, but- 1 
am sure that Solomon's few myrmecological comments, which have 
come down to us from about 970 B. C., are very accurate far 
more accurate than 'that story of Herodotus, written some 500 years 
later, of the gold-digging ants of India, which were as large as 
leopards, and whose hides were seen by Nearchus in the camp of 
Alexander the Great, and whose horns were mentioned by Pliny 
as hanging-, even in his time, in the temple of Hercules at Erythraa. 

i Lowell Lectures. 
Vol. XV.-25. 



386 THE SCIENTIFIC MONTHLY 

This and the many other ant stories invented or disseminated by 
ancient and modern writers are certainly not devoid of interest, 
but the actual behavior of the insect is so much more fascinating 
that you will pardon me for not dwelling on them. 

The. Formicidffi constitute the culminating group of the stinging 
Hymenoptera and have attracted many investigators for more' 
than a century and especially during the past thirty years. Unlike 
the honeybee these insects make no appeal to our appetites nor even 
to that vague affection which we feel for most of the common 
denizens of our forests, fields and gardens, but only to our in- 
quisitiveness and anxiety. Hence the vast literature which has 
been written on the ants may be said to have been prompted^ by 
scientific, philosophic or mere idle curiosity or by our instinc^of 
self-preservation'. In the presence of the ant we experience most 
vividly those peculiar feelings "which are aroused also by many 
other insects, feelings of perplexity and apprehension, which 
Maeterlinck has endeavored to express in the following words: 
' ' The- insect does not belong to our world. Other animals and even 
the plants, despite their mute lives, and the great secrets they enfold, 
seem not to be such total strangers, for we still feel in them, not- 
withstanding all- their peculiarities, a certain terrestrial fraternity. 
They may* astonish or even amaze us at times, but they do not 
completely upset our calculations. Something in the insects, how- 
ever, seems to be alien to the habits,, morals and psychology of our 
globe, as if it had come from some other planet, more monstrous, 
more energetic, more insensate, more atrocious, more infernal than 
our own. With whatever authority, with whatever fecundity, un- 
equalled here below, the insect seizes on life, we fail to accustom 
ourselves to the thought that it is an expression of that Nature 
whose privileged offspring we claim to be. ... No doubt, in 
this astonishment and failure to comprehend, we are beset with an 
indefinable, profound and instinctive uneasiness, inspired by be- 
ings so incomparably better armed and endowed than ourselves, con- 
centrations of energy and activity in which we divine our most mys- 
terious foes, the rivals of our last hours and perhaps our suc- 

v cessors. " . 

The similarities which the ants, as one of several families of 

,. aculeate or stinging Hymenoptera, necessarily bear to the wasps 
and bees, are so overlaid by elaborate specialization and idiosyn- 
crasies that their primitive vespine characters are not very easily 
detected. I wish to dwell on some of these specializations, but be- 
fore doing so, it will be advisable to give under separate captions 
a brief summary of what I conceive to be the fundamental pe- 
culiarities of the ants : 



SOCIAL LIFE AMONG THE INSECTS 387 

(1) The whole family Fonnicidje consists of social insects, 
that is, it includes no solitary nor subsocial forms such as we 
found among the beetles, wasps and. bees. We are therefore unable 
to point to any existing insects that might represent stages leading 
up to the social life of the ants. Within the family, iievertheless, 
we can distinguish quite a number of stages in a gradual evolution 
of social conditions from very simple, primitive forms, whose 
colonies consist of only a few dozen individuals, with a compara- 
tively feeble caste development, to highly specialized forms with 
huge colonies, comprising hundreds of thousands of individuals 
and an elaborate differentiation of castes. 

, (2) The number of described species of ants is approximately 
3,800, but if we include their subspecies and varieties, 'many of 
which will probably be raised to specific rank by future, less con- 
servativegenerations of entomologists; we shall have more than 
double the number. This is far in excess of the number of all 
other social insects, including both the groups I have already con- 
sidered and the termites. The ants are therefore the dominant 
social insects. 

(3) This dominance is shown also by .their geographical dis- 
tribution, which is world-wide. There are ants everywhere on the 
land-masses of the globe, except in high arctic and antarctic lati- 
tudes, and on the summits of the higher mountains. The number 
of individual ants is probably greater than that of all other insects. 
With few exceptions, the termites are all confined to tropical or 
^subtropical countries, and the number of social wasps and bees 
in temperate regions is very small. 

(4) We found that the social Wasps arose from the Eumenine 
solitary wasps and 'the bees from the solitary Sphecoids. All the 
authorities agree that the ants had their origin in neither of these 
ancestral stocks, but among the Scolioids, a distinct offshoot of the 
primitive Vespoids. Of .the four modern families of the Scolioids, 
the Psammocharidffi, Thynnidaj, Mutillidse and Scoliidas, the last 
seems to be most closely related to the ants. Since they must be 
traced to ancestors which were winged in both sexes, the Thynnids 
and Mutillids, which have wingless females, are excluded, and the 
family Psammocharida? is not very closely allied to the FormicidsE. 

(5) The ants, unlike the social wasps and bees, are eminently 
terrestrial insects. They inherited and seem very early to have 
exaggerated the terrestrial, habits of their primitive Scolioid ances- 
tors. The majority of the species in all parts of the world still 
nest in the soil. Many of them later took to nesting in dead or de- 
caying wood, and more recently a number of species, especially in 
the rain-forests of the 'tropics, have become, arboreal and nest by 
preference in the twigs of trees and bushes or construct paper or 



SOCIAL LIFE AMONG THE INSECTS 389 

or -sterile, caste, though distinctly differentiated, is, nevertheless, 
very much like the queen, or fertile female. In ants the differ- 
ences are much greater. Even when, as in many primitive ants 
(Fig. 54), the worker resembles the queen in size and form, .it 
never possesses wings, and in most ants the two castes are 'so dis- 
similar that they have often been described as separate species. 
The male ant, too, is much less like the queen than is the corre- 
sponding sex among the social wasps and bees (Fig. 57). It is 
evident, therefore, that all three castes are more highly specialized. 
In many ants, as we shall see, the -worker, queen and male may 
each become differentiated into two or more castes, a phenomenon 
which is nowhere even suggested among the wasps and bees. 

" ( 7) Very long and intimate contact with the soil has made the 
ants singularly plastic in their nesting habits. While most social 
wasps and bees construct elaborate combs with very regular, hexa- 
gonal cells of such expensive substances as paper and wax, the ants 
merely make more or less irregular galleries or chambers in the 
soil or dead wood or if they construct paper or silken nests avoid 
& rigid type of architecture. Hence the great variability of nest- 
ing habit 'in the same species. This plasticity and saving of time 
and labor are very advantageous, because they enable the insects, 
when conditions of temperature or moisture become unfavorable 
or when bothersome enemies settle too near the nest, to. change 
their habitation readily and without serious loss to the colony. 
Espinas long ago noticed the importance of the terrestrial habits 
of ants. .He says: "Ants owe their superiority to their terrestrial 
life. This assertion may seem paradoxical, but consider the excep- 
tional advantages afforded by a terrestrial compared with an 
aerial medium in the development of their intellectual faculties ! 
In the air there are the long nights without obstacles, the vertigin- 
ous journeys far from real bodies, the instability,, the wandering 
about, the endless f orgetfulness of things and of oneself. On the 
earth, on the contrary, there is not a movement that is not a con- 
.tact and does not yield precise information, not a journey that 
fails to leave some reminiscence ; and as these journeys are deter- 
minate, it is inevitable that a portion of the ground incessantly 
traversed should be registered, together with its resources and its 
dangers, in the animal's imagination. Thus there results a closer 
and much more direct communication with the external world. To 
.employ matter, moreover, is easier for a terrestrial than an aerial 
animal. When it is necessary to build, the latter must, like the 
bee, either secrete the substance of its nest or seek it at a distance, 
as does the bee when she collects propolis, or the wasp when she 
gathers material for her paper. The terrestrial animal has its 
building materials close at hand, and its architecture may be as 



390 THE SCIENTIFIC MONTHLY 

varied as these materials. Ants, therefore, probably owe their 
Social and industrial superiority to their habitat. " ' 

(8) The plasticity of ants is shown even more clearly in their 
care of their young, which are not reared in separate cells but in 
clusters and lie freely in the chambers and galleries of the nest 
where they can be moved about and easily carried away or hidden 
when the colony is disturbed or the moisture and temperature 
conditions are unfavorable. Like their continual contact with 
their physical environment, their intimate acquaintance with their 
young in all their stages has been an important factor in the high 
psychological development of the Formicidse. 

(9) A similar plasticity characterizes their feeding habits. As 
a group they feed on an extraordinary range of substances : ^he 
bodies and secretions of other insects, seeds, delicate fungi; nectar, 
the saccharine excreta of plant-lice, scale insects, etc. .Some species 
seem to be almost omnivorous. 

(10) All this adaptability, or plasticity in nesting and feeding 
habits is, of course, an expression of a very active and enterprising 
disposition and has resulted in the formation of a vast and intri- _ 
cate series of relationships between ants and other organisms, in-, 
eluding man. These restless, indefatigable, inquisitive busybodies, 
forever patrolling the soil and the vegetation in search of food, 
poke their noses, so to speak, into the private affairs of every living 
thing in their environment. Nor do they stop at this ; they actually 
draw many organisms, by domesticating them or at any rate at- 
taching them, to their nests or bodies, into the vortex of their ceast- 
less, impudent activities. Nearly every week during the past 
twenty years I have received from some entomologist somewhere 
on our planet one or more vials of ants with a request for their 
identification, often because they had been found associated with 
some insect or plant which the sender happened to be investi- 
gating. In the next lecture I shall describe a number of the 
strange partnerships into which ants have entered as a result of 
their inordinate and unappeasable appetites. 

As my time is limited I shall select for discussion only a few 
of the topics suggested in the foregoing summary, namely, the 
main taxonomic divisions of the family Formicidas, polymorphism, 
or the development of castes, the origin and growth of colonies, 
the structure of the alimentary canal in adult and larval ants and 
the evolution of the feeding habits. 

In their main outlines, at least, the phylogenetic relationships 
of the various subdivisions .or subfamilies of the Formicidee have 
been clearly established. There are seven of them: the Ponerinee, 
Cerapachyinas, Dorylinas, Pseudomyrminge, Myrmicinse, Dolicho- 



SOCIAL LIFE AMONG THE INSECTS 391 

FORMICINAE 
MY0M1CINAE 

DOUCHODERINAE 

A \IW \ \ . 

PSEUDOMYRMINAE 



CERAPACHYINAE 




DORYL1NAE 



SCOLIOID 
ANCESTORS. 

FIG. 55 

Ancestral tree showing the putative phylogenetic relations of the family 
Formici<te as a whole and of its subfamilies to one another. 

derinaa and Formiciii33. The Ponerinse constitute the primitive, 
basic stock of the family and have given rise to the six other sub- 
families, which are represented in the ancestral tree (Fig. 55) 
as so many branches. Their thickness roughly indicates their vigor 
or comparative development and their height their degree of spe- 
cialization and dominance in the existing fauna. All the sub- 
families are well represented in the tropics of both hemispheres, 
but in the north temperate region nearly all the species belong 
to the two .largest and highest subfamilies, the Myrmicinae and 
Formicinaa. In temperate North America and Eurasia there are 
very few Dolichoderinas and Ponerinee and no Cerapachyinas nor 
Pseudomyrmiiise. A small number of Dorylinae extend as far north 
as Colorado, Missouri and North Carolina (35 to 40) and to about 
the same latitude on the southern shores of the Mediterranean. 

With the exception of a series of peculiar parasitic genera, 
which are represented only by males and females, all ants possess a 
sharply denned worker caste. In primitive groups, like the 



392 



THE SCIENTIFIC MONTHLY 



Ponerinas, Cerapachyinse.and PseudomyrminiB, the worker is nearly 
as large as the queen but"la"cks.the wings and has therefore a more 
simply constructed thorax, the compound-.ey.es are smaller and the 
simple 'eye's, "or ocelli are minute or absent. In the three sub- 
families mentioned the worker is monomorph-ic, that is, it always 
has the same form though it may vary somewhat in size. In the 
four remaining subfamilies (Dory linse, Myr-micinee, Dolichoderinee 
and Formicinas) we find the same uniformity of the worker in 
many species";, but in a considerable number it has become, highly 
variable, or polymorphic, as a result of a'gencies which have acted 
independently in each subfamily, or even within the' limits of a 
single genus (Figs. 57 and 58). In such cases the workers can 
be arranged in a graduated series, beginning with large, 'huge- 
headed individuals more like the queen in stature, and ending with 
minute, small-headed individuals, which may 'be ,yery much smaller 
than the queen. ' Such a series exhibits not .only great morpholog- 




FIG. 57 

A small Myrmicine harvesting ant of Texas, Pheidole instabilis, with poly- 
morphic worker caste, a, soldier; /, worker; b to e, form's intermediate be- 
tween the soldier and. worker (lacking in most other species of the huge genus 
Pheidole) ; g, queen (dealated) ; h, male. The figures are all drawn to the 

same scale. 



394 THE SCIENTIFIC MONTHLY 

where soldiers were not needed or were too expensive to rear and 
maintain, on account of their great size and appetites, they too 
have been eliminated and the worker caste is represented only by 
the tiniest individuals of the originally polymorphic series (Care- 
bara, Tranopelta, Psedalgus, Solenopsis, etc.). There is therefore 
an enormous difference in these ants in size -and structure between 
. the queen and the. only surviving worker form of the species. In 
Carebara, e. g., the queen is several thousand times as large as 
the worker ! Nevertheless, both are merely extreme female forms 
of the same species and may, of course, develop from the 'eggs of 
the same mother. ' ' 

But the worker is not the only caste that has become dimorphic. 
In some species there, are two distinct forms of queen, in others 
two distinct forms of male. In these cases one of the forms is 
winged, the other usually apterous. And here again, by suppres- 
sion of the winged female or winged male, the wingless form may 
become the only surviving fertile form of its sex in the species. 
All these developments -are interesting because they indicate that 
the distinctions among the. various castes have arisen gradually 
by continuous or fluctuating variations and that the survival and 
persistence of some of them and the elimination of others have led 
to the sharply discontinuous series of castes which we find in 
many ants. 

It is obvious that some of the differences between the various 
castes, especially those in size, are due to differences in the amount 
of food consumed during the larval stages, but the profounder 
morphological differences which separate the queens, 'soldiers and 
workers, must be due to other causes. We must suppose either 
that the food administered to the larva? differs in quality or that 
there are several different kinds of eggs, some of which develop 
into fertile, other into sterile forms. In a sense the latter would be 
mutations, like the various sterile forms of the evening primrose, 
which make their appearance generation after generation from 
some of the seeds of the fertile forms. In the case . of the ants, 
however, we find that the workers not infrequently lay viable eggs, 
and though they are never fertilized and 'generally develop into 
males, the latter may mate with queens and thus be a means of 
establishing a representation of the characters of their worker 
mothers in the germ-plasm of the species. The peculiar anomalies 
known as gymandromorphs, that is, individuals partly male and 
partly female, which occasionally occur among ants, also indicate 
that the queens, soldiers and workers arise from as many different 
kinds of eggs, since there are three different kinds of gynandro- 
morphs, exhibiting respectively combinations or mosaics of male 
and queen, male and soldier and male and worker characters. It 



SOCIAL LIFE AMONG THE INSECTS 395 

is difficult to see how such perfectly definite combinations could 
be produced by larval feeding, and it is equally difficult to account 
for them as the results of internal secretions. In the present state 
of our knowledge we can only surmise that the differences between 
the queen and worker castes were originally ontogenetic and de- 
termined by feeding, as they still are in the social wasps and bees, 
but that in the ants the germ-plasm has somehow been reached 
and modified, so that an hereditary basis for caste differentiation 
has been established. 

The ant colony may be initiated and developed by one of two 
different methods which I shall call the independent and the de- 
pendent. The former is peculiar to the nonparasitic, the latter to 
the, parasitic species. Leaving an account of the ants which employ 
the dependent method for the next lecture, I would say that the 
great majority of ants establish their colonies in essentially the 
same manner as Vespa and the bumble-bees. The winged, virgin 
queen, after fecundation during her nuptial flight, descends to 
the ground, rids herself of her wings and seeks out some small 
cavity under a stone or piece of bark, or excavates a small cell in 
the soil. She then closes the opening of the cell and remains a 
voluntary prisoner for weeks or. even months while the eggs are 
growing in her ovaries. The loss of the wings has a peculiar effect 
on the voluminous wing-muscles in her thorax, causing them to 
break down and dissolve in the blood plasma: Their substance is 
carried by the circulation to the ovaries and utilized in building 
up the yolk of the eggs. As soon as the eggs mature, they are laid 
and the queen nurses the hatching larvffi and feeds them with her 
saliva till they ptrpate. Since she never leaves the cell during all 
this time and has access to no food, except the fat she stored in 
her abdomen during her larval life and her dissolved wing-muscles, 
the workers that emerge from the pupaa are all abnormally small. 
They are, in fact, always minimse in species which have a poly- 
morphic worker caste. They dig their way out through the soil, 
thus establishing a communication between the cell and the outside 
world, collect food for themselves and their mother and thus enable 
her to lay more eggs. They take charge of the second brood of 
eggs and larva, which, being more abundantly fed, develop into 
larger workers. The population of the colony now increases 
rapidly, new chambers and galleries are added to the nest and the 
queen devotes herself to digesting the food received from the 
workers and to laying more eggs. In the course of a few years 
numerous males and queens are reared and on some meteorologic- 
ally favorable day the fertile forms from all the nests of the same 
species over a wide expanse of country escape simultaneously into 
the air and celebrate their marriage flight. This flight provides 



396 THE SCIENTIFIC MONTHLY 

not only for the mating of the sexes but also for the dissemina- 
tion of the species, since the daughter queens, on descending, to 
the ground, usually establish their nests at some distance from the 
parental colony. 

It will be seen that the queen ant, ' like the queen wasp and 
bumble-bee, but unlike the queen honeybee, is the perfect female 
of her species, possessing not only great fecundity but in addition 
all the worker propensities, as shown by her ability to make a nest 
and bring up her young.' But as soon as the first brood of workers 
appears, . these propensities are. no longer manifested. That they 
are not lost is shown by the simple experiment of' removing . the 
queen's first brood of -workers. 'Then, provided she be fed or have 
a. sufficient store of food in her body, she will at once proceed .to 
bring up another brood in the same manner as the first, although 
she would have manifested no such behavior under normal con- 
ditions. 

As already stated, this independent method of colony forma- 
tion is the most universal and is followed alike by tropical and 
extratropical ants. It is undoubtedly the primitive method and, 
as we shall see, the one from which the dependent method has been 
derived. It differs from that of Vespa and Bombus, nevertheless, 
in leading to the formation of perennial colonies even in temperate 
and boreal regions. The queen ant may, in fact, live from 12 to 
17 years and although, like other aculeates, she is fecundated only 
once, may produce offspring up to the time of her death. Unlike 
the queen honeybee she is never hostile to her own queen daughters, 
and in many species of ants some of these daughters may return 
after their marriage flight to the maternal colony and take a very 
active part in increasing its population. In this manner the colony 
may become polygynic or pleometrotic, and in some instances may 
contain a large number of fertile queens. When such a colony 
grows too large it may separate into several, the queens emigrating 
singly or in small companies, each accompanied by a detachment 
of workers, to form a new nest near the parental formicary. This 
behavior is exhibited by the well-known mound-building ant 
(Formica .exsectoides} of our New England hills. You will notice 
that its mounds usually occur in loose groups or clusters and that 
the workers of the different nests are on friendly terms with one 
another and sometimes visit back and forth. We may, of course, 
call the whole cluster a single (polydomous) colony, but it really 
differs from a number of colonies only in the absence of hostility 
between the inhabitants of the different mounds. , In certain 
tropical ants, like the Dorylinffi (Figs. 59 and 60), however, I am 
inclined to believe that the only method of colony formation is 
by a. splitting of the original colony into as many parts as it con- 



398 



THE SCIENTIFIC MONTHLY 



tains young queens. These huge, clumsy creatures (Fig. 60) are 
always wingless and must therefore be fecundated in the nest, and 
since the colonies, which comprise hundreds of thousands of work- 
.ers, are nomadic and keep wandering from place to place, they 
mus.t become independent entities as soon as they are formed. 

We possess no accurate data on the age that ant colonies may 
attain. Some of them certainly persist for 30 or 40 years and 
probably even longer. In such old colonies the original queen has, 
of course, been replaced by successive generations of queens, that 
is, by her fertile daughters, grand-daughters and great-grand- 
daughters, and the worker personnel has been replaced at a more 
rapid rate, because the individual worker does not live more, and 
in most instances lives considerably, less, than three or four years. 

The. feeding habits of ants are so va.ried and complicated that 
it will be advisable before considering them to describe the struc- 
ture of the alimentary canal in both adult and larva. The mouth- 
parts of the adult are of the generalized vespine type and consist 

A 




FIG. 6l . ' 

Sagittal sections through the heads of ants. A, of queen Lasius niger with 
the mouth open (After Janet). B, of queen Camponotus brutus.with the 
mouth closed, t, tongue; o, oral orifice; ph, pharynx; h,. infrabuccal pocket; 
pe, pellet in situ, made up of solid particles of food refuse and^strigil sweep- 
ings. Note stratification in the substance of the. pellet, indicating successive 
meals or toilet operations. 



SOCIAL LIFE AMONG THE INSECTS 399 

of a small, flap-like upper lip, or labrum, a pair of strong, usually 
toothed mandibles, a pair of small maxillas and a broad lower lip, 
or labium. The maxillas and labium are each provided with a pair 
of jointed, sensory appendages, the palpi. The mandibles, which 
are reallythe ant's hands, vary greatly in shape in different genera 
and are used not only in securing the food but also in many other 
activities, such as digging in the earth or wood, transporting other 
ants or the young, fighting, leaping, etc. ' Liquids are, of course, 
merely imbibed and swallowed, but solid food is seized and crushed 
with the mandibles and the juices or smaller particles licked up 
with the tongue, which is a roughened pad at the tip of the lower 
lip (Fig. 61i) just anterior to the opening of the duct of the 
salivary glands. The small particles thus collected are carried 
back into a small chamber or sac, the infrabuccal pocket (Pig. 61/0, 
which lies immediately below and anteriorly to the mouth-open- 
ing (0). This pocket is an important structure since it serves as 
a receptacle not only for the more solid particles of food but also 
for the dirt, fungus-spores, etc., which the ant collects during her 
toilet operations, for the ant is an exquisitely cleanly insect and 
devotes much of her leisure to licking and burnishing her own 
smooth or finely chiseled armor and that of her nest-mates. More- 
over, the tip of the fore tibia is furnished with a beautiful comb 
or strigil which can be opposed to another comb on the concave 
inner surface of the fore metatarsus. The ant cleans her legs and 
antennae by drawing them between these . combs, which are then 
drawn across the mouth, with the result that any adhering dirt 
is carried off into the infrabuccal pocket. In this manner the dirt 
and the solid or semisolid food particles are combined and the 
whole mass moulded in the infrabuccal pocket into the form of a. 
roundish oblong pellet (Fig. 61B pe). After any liquid which it 
may contain has been dissolved out and sucked back into the 
mouth, the pellet is cast out, so that no solid food actually enters 
the alimentary canal. All adult ants therefore subsist entirely 
on liquids. 

The alimentary canal proper is a long tube extending through 
the body and divided into sections, each with its special function. 
The more anterior sections are the mouth cavity, the pharynx 
(Fig. 61 ph), which receives the ducts of certain glands, and the 
very long, slender gullet, which traverses the posterior part of the' 
head, the whole thorax and the narrow waist, or pedicel of the 
abdomen as far as the base of its large, swollen portion, the gaster. 
Here the gullet expands into a thin-walled, distensible sac, the 
crop, which is used for the storage of the imbibed liquids. At 
its posterior end the crop is separated from the ellipsoidal stomach 
by a peculiar valvular constriction, the proventriculus. The 
hindermost sections of the alimentary tract are the intestine and 



400 THE SCIENTIFIC' MONTHLY 

the large, pear-shaped rectum.- The crop, pro.ventriculus and 
stomach are the most . interesting of these various organs. . Forel 
calls the crop the "social stomach," because its. liquid contents 
are in great part distributed by regurgitation to the other members 
of the 1 colony and because only a small portion, which is permitted 
to pass back through the proventricular valve and enter the stom- 
ach, is absorbed and utilized by the individual ant. That the crop 
' functions in the manner described can be readily demonstrated by 
permitting some pale yellow worker ant to gorge herself with syrup 
stained blue or . red with an aniline dye. The ant's gaster will 
gradually become vividly colored as the crop expands. Now if the 
insect be' allowed to return to the nest, other workers will come up 
to it, beg for food with rapidly -vibrating antennas and protrude 
their tongues, and very soon their crops, too, will become visible 
through the translucent -gastric integument as they fill with the 
stained syrup. Then these workers in turn will distribute the 
food 'by regurgitation in the same manner till every member of 
the colony has at 1 least' a minute share of the blue or red cropful 
of the first worker, 

The alimentary tract of the helpless, legless, soft-bodied ant 
grub or larva is much simpler than that of the adult. The mouth- 
parts are similar but more rudimentary. As a, rule, the mandibles 
are less developed but 'in some larvae they are strong, dentate and 
very sharp. The lower lip is fleshy and'protrusible and provided 
with sensory papillas instead of palpi, and the -unpaired duct of 
the long, tubular and more or less 'branched salivary glands opens 
near its tip. The mouth-opening is broad .and its lining in many 
species is provided with numerous transverse ridges beset with 
very minutes spinules (Fig. '62C). Larger,, pointed projections 
or imbrications may also cover the basal portions of the mandibles. 
All these spinules and projections are probably used in triturating 
the food but perhaps when rubbed on one another they may also 
produce shrill sounds for the purpose of apprising the worker 
nurses of the hunger or discomfort of their charges. The gullet 
'is long and very slender and opens directly into the .large stomach, 
which throughout larval life is closed behind, that is,, does not open 
into the intestine. A communication with' the more posterior por- 
tion of the alimentary tract- is not established till the larva is about 
to pupate. Then all the undigested food which has accumulated 
in the stomach since the very beginning of larval life is voided as 
a large black pellet, the meconium. 

In the larvas of the Pseudomyiminae (Figs. 62, 63 and 64). there 
are certain very peculiar additional structures which may be briefly 
described. The head is' not at the anterior end of the body as in. 
other ant larva but pushed far back on the ventral surface so that 
'it is surrounded' by a great hood formed from the three thoracic 



SOCIAL LIFE AMONG THE INSECTS 



401 




FIG. 62 

Larva of Pseudoni-yrtna gracilis. A, ventral; B, lateral view; C, head and 
adjacent portions of same enlarged; D, sagittal section through anterior por- 
tion of larva, o, oral orifice; x, exudatoria; *, trophothylax, or pocket, which 
holds the pellet; (pe~), deposited by the worker nurses and which is eaten by 
the larva. Note the hooked dorsal hairs of the larva, which serve to suspend 
it from the walls of the nest. . a, mouth cavity, more enlarged to show the 
fine spinules (also seen in C), which serve to triturate the pellet and probably 
also as a stridulatory organ. 

Vol. XV.-26. 



402 



THE SCIENTIFIC MONTHLY 




FIG. 63 

A, ventral; B, lateral view of the first larval stage ("trophidium") of the 

Ethiopian Pachysima lattfrons, -showing the peculiar appendages ("exuda- 

toria") surrounding the head. These belong to the three thoracic and the 

first abdominal segments. 

segments, and the first abdominal segment, which lies immediately 
behind the head, has in the midventral line a singular pocket, the 
trophothylax (t). Furthermore, each side of this segment and 
each ventrolateral portion of the -several thoracic segments is de- 
veloped as a peculiar protuberance or appendage, which functions- 
as a blood-gland, or exudatorium (x). 

Unlike the adult ants the larva; can devour solid food, though 
they are often fed, at least in their youngest stages, with liquids 
regurgitated on their mouths by the worker nurses. The-larvse of 
the PseudoruyrminEe . are fed with the pellets (pe) from the in- 
frabuccal pocket, which are placed by the workers in the tropho- 
thylax where they are within easy reach of the mandibles and can 
be gradually drawn into the mouth, triturated and swallowed. 
Some primitive ants (Ponerinffi, some Myrmicinas, etc.) actually 
feed their young with pieces of insects or entire small insects, 
which are simply placed on the ventral surface of the larva within 
reach of its mouth-parts. 

In a former lecture I referred to the fact that the larvse of 
the social wasps, either before or after feeding, produce droplets 
of a sweet salivary secretion, which are eagerly imbibed by the 
adult wasps, and I designated this interchange of food between 
adult and larva as trophallaxis. I have recently made some ob- 
servations which show that the . ant larvse also produce secretions 
which appeal to the appetites of their nurses. These secretions 
are more varied than in the wasps. Certain ant larvee undoubtedly 



SOCIAL LIFE AMONG THE INSECTS 



403 



supply their nurses with saliva, but many or all sweat a fatty 
secretion through the delicate general integument of the body, 
and the larval PseudomyrmiiijE produce similar exudates from 
the papillaa or appendages above described. Although these various 
substances are produced in very small quantities they are of such 
qualities that they are eagerly sought by the adult ants. This 
explains much of the behavior which has been attributed to ma- 
ternal affection on the part of the queen and the workers, such 
as the continual licking and fondling of the larva, the ferocity 
with which they are defended and the solicitude M'ith which they 
are removed when the nest is disturbed. In other words, a de- 
cidedly egoistic appetite, and not a purely altruistic maternal 
anxiety for the welfare of the young constitutes the potent ' ' drive ' ' 
that initiates and sustains the intimate relations of the adult ants 
to the larvaa, just as the mutual regurgitation of food initiates and 
sustains the similar relations among the adult workers themselves. 
I am convinced that trophallaxis will prove to be the key to 
an understanding not only of the behavior I have briefly outlined 




FIG. 64 

Second, third and fourth (adult) larval stages of Pachysima latifrons, show- 
ing the gradual dwindling of the exudatoria. A and B show the trophothy- 
lax (t) ; and B also shows the food pellet pe; which is the pellet formed in the 
infrabuccal pocket of the worker nurse; x, exudatorium. See Figs. 62 and 63. 



404 



THE SCIENTIFIC MONTHLY 



but also of the relations which ants have acquired to many kinds 
of alien organisms. In the accompanying diagram (Fig. 65) I 
have endeavored to indicate how trophallaxis, originally developed 
as a mutual trophic relation between the queen ant and her brood, 
has expanded with the growth of the colony, like an ever-widening 
vortex, till it involves, first, all the adults as well as the brood 




FIG. 65 

See text for explanation, 

and therefore the entire colony; second, a great number of alien 
insects that- have managed to 'get a foot-hold in the nest as 
scavengers, predators and parasites (symphiles) ; third, alien social 
insects, that is, other species of ants (social parasites) ; fourth 
alien insects that live outside the nest and are "milked" by the 
ants (trophobionts), and fifth, certain plants . that are regularly 
visited or even inhabited by the ants (myrmeeophytes). These 
extranidal relationships, represented by the two outer rings in 
the diagram are, -of course, incomplete or , one-sided, since the 
organisms which they represent are not fed but merely cared for or 
protected by the ants. ' -In my next lecture I shall have more to 

say about some of these relationships. 

(To l>e continued)